TM 5-811-14
CHAPTER 3
OVERCURRENT PROTECTIVE DEVICES
3-1. General
Design of power system protection requires the
proper application motor of overload relays, fuses,
circuit breakers, protective relays, and other special
purpose overcurrent protective devices. This
chapter provides detailed information about various
protective devices, illustrates their time-current
characteristics, and identifies information required
to design coordinated power system protection.
3-2. Motor overload relays
a. Thermal overload relays. The most common
overcurrent protective device is the thermal overload relay associated with motor starting contactors. In both low-voltage and medium-voltage
motor circuits, thermal overload relays detect
motor overcurrents by converting the current to
heat via a resistive element. Thermal overload
relays are simple, rugged, inexpensive, and provide
very effective motor running overcurrent protection. Also, if the motor and overload element are
located in the same ambient, the thermal overload
relay is responsive to changes in ambient temperature. The relay trip current is reduced in a high
ambient and increased in a low ambient. Typical
time-current characteristic curves for thermal
overload relays are shown in appendix C. The
curves level off at about 10 to 20 times full-load
current, since an upstream short-circuit device,
such as a fuse or circuit breaker, will protect the
motor circuit above these magnitudes of current.
The thermal overload relay, therefore, combines
with the short-circuit device to provide total overcurrent protection (overload and short-circuit) for
the motor circuit.
(1) Melting alloy type overload relays, as the
name implies, upon the circuit when heat is sufficient to melt a metallic alloy. These devices may be
reset manually after a few minutes is allowed for
the motor to cool and the alloy to solidify.
(2) Bimetallic type overload relays open the
circuit when heat is sufficient to cause a bimetallic
element to bend out of shape, thus parting a set of
contacts. Bimetallic relays are normally used on
automatic reset, although they can be used either
manually or automatically.
(3) Standard, slow, and quick-trip (fast) relays
are available. Standard units should be used for
motor starting times up to about 7 seconds. Slow
units should be used for motor starting times in the
8-12 second range, and fast units should be used on
special-purpose motors, such as hermetically sealed
and submersible pump motors which have very fast
starting times.
(4) Ambient temperature — compensated overload relays should be used when the motor is located in a nearly-constant ambient and the thermal
overload device is located in a varying ambient.
b. Magnetic current overload relays. Basically,
magnetic current relays are solenoids. These relays
operate magnetically in response to an overcurrent. When the relay operates, a plunger is
pulled upward into the coil until it is stopped by an
insulated trip pin which operates a set of contacts.
Magnetic relays are unaffected by changes in
ambient temperature. Magnetic current relays may
be used to protect motors with long starting times
or unusual duty cycles, but are not an alternative
for thermal relays.
c. Information required for coordination. The
following motor and relay information is required
for a coordination study.
(1) Motor full-load ampers rating from the
motor nameplate.
(2) Overload relay ampere rating selected in
accordance with NFPA 70.
(3) Overload relay time-current characteristic
curves.
(4) Motor locked rotor amperes and starting
time.
(5) Locked rotor ampere damage time for
medium-voltage motors.
3-3. Fuses
A fuse is a non-adjustable, direct acting, singlephase device that responds to both the magnitude
and duration of current flowing through it. Fuses
may be time delay or non-time delay, current-limiting or non-current-limiting, low-voltage or highvoltage. Fuse terminology and definitions are listed
in the glossary. Underwriter’s Laboratories (UL)
further classifies low-voltage fuses as shown in
table 3-1.
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a. Nontime delay fuses. The Nontime delay fuse
consists of a single type of fusible element, called a
short-circuit element. Normal overloads and current surges often cause nuisance openings of this
type of fuse. For this reason, substantial oversizing
of these fuses is required when used in motor circuits. Therefore, Nontime delay fuses should be
used only in circuits with noninductive loads such
as service entrances and circuit breaker back-up
protection.
b. Time delay fuses. The time delay fuse is constructed with two different types of fusible elements: overload and short-circuit. These elements
are somewhat similar in operation to the thermal
and magnetic elements of an inverse-time circuit
breaker. The overload element will interrupt all
overload currents, and the short-circuit element will
open in response to short-circuit currents. The time
delay fuse can be applied in circuits subject to
normal overloads and current surges (e.g., motors,
transformers, solenoids, etc.) without nuisance
opening. Significant oversizing is not necessary.
c. UL classification. As shown in table 3-1, UL
has established distinct classifications for low-voltage fuses. These classifications define certain operating characteristics associated with a particular
fuse class. However, the fact that a fuse is classified, for example, as UL RK-5 does not mean that
all of its operating characteristics are identical with
those of other manufacturers’ Class RK-5 fuses.
Both time delay and Nontime delay fuses are
classified as RK-5. Therefore, each type of RK-5
fuse will require different application procedures.
UL classifications and time delay characteristics
should always be specified along with current and
voltage ratings for low-voltage fuses. This will
eliminate any confusion on the Contractor's part
and insure that the correct fuse is always provided.
UL Class H fuses are tested for short-circuit ratings
at 10,000 amperes symmetrical, and are, therefore,
not current-limiting. UL Class K fuses are tested at
50,000, 100,000, or 200,000 amperes symmetrical,
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and are, therefore, current-limiting. However, Class
K fuses are not labeled as current-limiting, because
Class K fuses are interchangeable with UL Class H
and NEMA Class H fuses. Therefore, Class H and
Class K fuses should be avoided in favor of Class
RK and Class L fuses. Rejection-type fuses and
fuse holders prevent underrated fuses from
inadvertently being installed.
d. Time-current characteristic curves. Fuse
curves are available from various manufacturers.
'Typical fuse time-current characteristic curves are
shown in appendix C. Medium-voltage and highvoltage fuses show an operating "band" while lowvoltage fuses show an operating “line.” The band
associated with medium-voltage and high-voltage
fuses graphically displays minimum melting time
and total (or maximum) clearing time as a function
of current magnitude.
e. Current-limitation. Current-limiting fuses
are so fast acting that they are able to open the
circuit and remove the short-circuit current well
before it reaches peak value. Current-limiting fuses
“limit” the peak short-circuit current to a value less
than that available at the fault point and open in less
than one-half cycle. To be effective, however, such
fuses must be operated in their current-limiting
range. Peak let-through charts, also called currentlimiting effect curves, should be used to determine
the effectiveness or degree of protection offered by
current-limiting fuses. These curves plot
instantaneous peak let-through current as a function
of available RMS symmetrical short- circuit current.
f. Medium-voltage fuses. Medium-voltage fuses
are either (1) distribution fuse cutouts or (2) power
fuses. Distribution fuse cutouts are designed for
pole or crossarm mounting and should be used primarily on distribution feeders and circuits. Power
fuses have a higher dielectric strength than distribution fuse cutouts and should be used primarily in
substations. The majority of medium-voltage fuses
are used for applications within buildings, vaults, or
enclosures. They are boric acid type fuses rated
4160—34.5kV or current-limiting fuses rated
2400V—34.5kV.
g. High-voltage fuses. Some medium-voltage
fuses and all high-voltage fuses are rated for outdoor use only. These devices are boric acid type
fuses rated 4160V—138kV, fiberlined expulsion
fuses rated 7200V—l6lkV, or distribution fuse cutouts rated 4800V—138kV.
h. Current-limiting power fuses. Current-limiting power fuses include E-rated, C-rated, and Rrated fuses. E-rated, current-limiting, power fuses
rated 100E and below open in 300 seconds at currents between 200 percent and 240 percent of their
E-rating. Fuses rated greater than 100E open in 600
TM 5-811-14
seconds at currents between 220 percent and
264 percent of their E-rating. C-rated, current-limiting power fuses open in 1000 seconds at currents
between 170 percent and 240 percent of their Crating. R-rated, current-limiting power fuses are
suitable for use on medium-voltage motor controllers only. Generally, R-rated fuses open in 20 seconds at 100 times the R-rating.
I. Information required for coordination. The
following fuse information is required for a coordination study:
(1) Fuse continuous current rating.
(2) Fuse time-current characteristic curves.
(3) Fuse interrupting-current rating.
(4) UL classification and time delay characteristics.
j. Fuse ratings. Standard voltage and current
ratings for fuses can be found in appendix D.
3-4. Motor short-circuit protectors (MSCP)
Motor short-circuit protectors are current-limiting,
fuse-like devices designed specifically for use in
switch-type, combination motor controllers. UL
considers MSCPs to be components of motor controllers rather than fuses. Therefore, MSCPs are
marked by letter designations (A-Y) instead of
ampere ratings and may not be used as fuses.
MSCPs may be used in motor circuits provided the
MSCP is part of a combination motor controller
with overload relays and is sized not greater than
1,300 percent of motor FLA (NFPA 70). This relatively new arrangement (first recognized by NFPA
70-1971), provides short-circuit protection, overload protection, motor control, and disconnecting
means all in one assembly. MSCPs provide excellent short-circuit protection for motor circuits as
well as ease of selection. However, the limited
number of manufacturers that can supply MSCPs
has so far prohibited their use by the Government
except for sole-source applications.
3-5. Circuit breakers
A circuit breaker is a device that allows automatic
opening of a circuit in response to overcurrent, and
also manual opening and closing of a circuit.
Circuit breaker terminology and definitions are
listed in the glossary. Low-voltage power circuit
breakers have, for years, been equipped with electromechanical trip devices. Modern, solid-state devices, however, are rapidly replacing electromechanical trips. Solid-state trips are more accessible,
easier to calibrate, and are virtually unaffected by
vibration, temperature, altitude, and duty-cycle.
Furthermore, solid-state devices are easy to
coordinate, and provide closer, more improved protection over electromechanical units. Still, electromechanical units have their applications. Industrial
plants with harsh environments, such as steel mills
and ammunition plants, may demand the more
rugged electromechanical devices. Today, moldedcase circuit breakers are being equipped with solidstate trip units to obtain more complex tripping
characteristics. Surface-mount, or integratedcircuit, technology is allowing very sophisticated
molded-case circuit breakers to be constructed in
small frame sizes. Most low-voltage power circuit
breakers are also being equipped with solid-state
trip units. New microprocessor-based circuit
breakers are now available that offer true RMS
current sensing. The increased use of switchingmode power supplies for computer systems and
other harmonic-generating, non-linear loads created
the need for true RMS sensing, which is a major
advantage over peak-sensing trip units.
a. Low-voltage circuit breakers. Low-voltage
circuit breakers are classified as molded-case circuit
breakers or power circuit breakers. A molded-case
circuit breaker is an integral unit enclosed in an
insulated housing. A power circuit breaker is
designed for use on circuits rated 1000 Vac and
3000 Vdc and below, excluding molded-case circuit
breakers.
(1) Low-voltage circuit breaker trip units may
be of the electromechanical (thermal-magnetic or
mechanical dashpot) or solid-state electronic type.
Low-voltage circuit breakers may include a number
of trip unit characteristics. These characteristics are
listed below and illustrated in figures 3-1 and 3-2.
Typical circuit breaker time-current characteristic
curves may be found in appendix C. Circuit breaker
curves are represented as "bands." The bands
indicate minimum and maximum operating times for
specific overcurrents.
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3-4
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3-5
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(a) Long-time pick-up allows fine tuning of
the continuous current rating. Typical settings
range from 50 percent-100 percent of circuit breaker sensor current rating.
(b) Long-time delay varies the tripping time
under sustained overcurrent and allows momentary
overloads. Three to six bands are typically
available.
(c) Short-time pick-up controls the amount
of high-level current that can be carried for short
periods of time without tripping and allows downstream devices to clear faults without tripping upstream devices. Typical settings range from 1.5 to
9 times long-time pick-up setting.
(d) Short-time delay is used with short-time
pick-up to improve selectivity. It provides time
delay to allow the circuit breaker to trip at the selected short-time pick-up current. Three bands
(minimum, intermediate, and maximum) are typically available.
(e) Short-time I2t switch introduces a ramp
function into the short-time characteristic curve to
improve coordination with downstream devices
whose characteristic curves overlap the circuit
breaker characteristic curve.
(f) Instantaneous pick-up establishes the
tripping current level with no intentional time delay.
Typical settings range from 1.5 to 9 times Longtime pick-up setting.
(g) Ground-fault pick-up establishes ground
fault tripping current level and may incorporate the
I2 t function. Ground-fault pick-up is typically
adjustable from 20 percent to 100 percent of sensor
rating. Ground-fault pick-up should never be set
above 1200 A in accordance with NFPA 70.
(h) Ground-fault delay incorporates time
delay for coordination. Three to six time delay
bands are typically available. Ground-fault delay
should not exceed one second for ground-fault currents greater than 3000 A in accordance with
NFPA 70.
(2) Specifications should detail only those functions that are necessary on a particular project.
(a) The continuous current rating may be
fixed or adjustable.
(b) Molded-case breakers with solid-state
trips and power breakers normally have adjustable
long-time and short-time functions.
(c) Power breakers may or may not have the
instantaneous function.
(d) Most molded-case circuit breakers, especially in the smaller sizes, are not provided with
long-time adjustments, short-time functions, or
ground-fault functions.
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(3) The inverse-time (or thermal-magnetic) circuit breaker contains a thermal and a magnetic
element in series and is similar in operation to time
delay fuses. This circuit breaker will trip thermally
in response to overload currents and magnetically
in response to short-circuit currents. Magnetic
tripping is instantaneous while thermal tripping
exhibits an inverse-time characteristic (i.e., the
circuit breaker operating characteristics of time and
current are inversely proportional). Inverse-time
circuit breakers have three basic current ratings:
trip rating, frame rating, and interrupting rating.
Trip rating is the minimum continuous current
magnitude required to trip the circuit breaker
thermally. The frame rating identifies a particular
group of circuit breakers and corresponds to the
largest trip rating within the group. Each group
consists of physically interchangeable circuit
breakers with different trip ratings, as shown in
table 3-2. Although NEMA recognizes other frame
ratings in addition to those listed in table 3-2, these
are the most common ones supplied by
manufacturers. The interrupting rating describes the
short-circuit withstand capability of a circuit
breaker.
(4) The instantaneous-trip circuit breaker is
nothing more than an inverse-time circuit breaker
with the thermal element removed and is similar in
operation to the non-time delay fuse. This circuit
breaker is often referred to by other names, such as,
magnetic circuit breaker, magnetic-only circuit
breaker, or motor circuit breaker. Instantaneoustrip circuit breakers may be used in motor circuits,
but only if adjustable, and if part of a circuit breaker
type, combination motor controller with overload
relays. Such an arrangement is called a Motor
Circuit Protector (MCP) and provides short-circuit
protection (circuit breaker magnetic element),
overload protection (overload relays), motor
control, and disconnecting means all in one
assembly. Instantaneous-trip circuit breakers have
frame and interrupting ratings but do not have trip
ratings. They do have an instantaneous current
rating which, for motor circuits, must be adjustable
TM 5-811-14
and not exceed 1,300 percent of the motor FLA
(NFPA 70). MCPs provide excellent motor circuit
protection and ease of specification, and should be
considered for installations with numerous motors
where MCCs would be specified.
(5) A current-limiting circuit breaker does not
employ a fusible element. When operating within its
current-limiting range, a current-limiting circuit
breaker limits the let-through I2t to a value less than
the I2 t of the quarter cycle of the symmetrical
current. Current-limiting circuit breakers employ
single and double break contact arrangements as
well as commutation systems to limit the letthrough current to satisfy the fundamental definition of current-limitation without the use of the
fuses. Current-limiting circuit breakers can be reset
and service restored in the same manner as
conventional circuit breakers even after clearing
maximum level fault currents. Manufacturers of
current-limiting circuit breakers publish peak letthrough current (Ip) and energy (I2t curves. The
manufacturer should be contacted for specific application information.
(6) Integrally fused circuit breakers employ
current limiters which are similar to conventional
current-limiting fuses but are designed for specific
performance with the circuit breaker. Integrally
fused circuit breakers also include overload and low
level fault protection. This protection is coordinated so that, unless a severe fault occurs, the current limiter is not affected and replacement is not
required. Current limiters are generally located
within the molded case circuit breaker frame. An
interlock is provided which ensures the opening of
the circuit breaker contacts before the limiter cover
can be removed. Single phasing is eliminated by the
simultaneous opening of all circuit breaker poles.
Many circuit breakers employ mechanical interlocks
to prohibit the circuit breaker from closing with a
missing current limiter. The continuous ampere
rating of integrally fused circuit breakers is selected
in the same manner as for conventional circuit
breakers. The selection of the individual limiters
should be made in strict accordance with the
manufacturer's published literature to achieve the
desired level of circuit protection.
(7) A molded-case circuit breaker can be applied in a system where fault current may exceed its
rating if it is connected in series on the load side of
an acceptable molded-case circuit breaker. Such an
application is called cascade system operation. The
upstream breaker must be rated for maximum
available fault current and both breakers must be
tested and UL certified for a series rating. Cascade
operation depends upon both breakers opening at
the same time, and upon the fact that the upstream
breaker will always open. Since molded-case
circuit breaker contacts are designed to "blow
open" on high short-circuit currents, failure of the
upstream breaker to operate is not a concern. Since
low-voltage power breakers are not designed to
"blow open," power breakers should not be applied
in cascade. Individual components within a cascade
system should not be replaced since the entire
system is UL approved. Individual components are
not UL approved. Additionally, individual
components should be from the same manufacturer
as the cascade system. By virtue of the design, this
approach does not pro- vide a coordinated system.
b. Medium-voltage circuit breakers. ANSI defines medium-voltage as 1000V or more, but less
than 100kV. Switching a medium-voltage circuit
involves either opening or closing a set of contacts
mechanically. When closing the contacts, the applied mechanical force must be greater than the
forces which oppose the closing action. An arc is
created when the contacts are opened, which must
be extinguished. Medium-voltage circuit breakers
are classified according to the medium (oil, air,
vacuum, or SF6) in which their contacts are immersed. Normally, metal clad, drawout switchgear
is used at medium-voltages up to 15kV. Air-magnetic, vacuum, and SF6-filled-interrupter circuit
breakers are available in drawout switchgear. Oil
circuit breakers are used outdoors, as individual
units, and thus are not available in drawout
switchgear mounting.
(1) Medium-voltage air circuit breakers are
either of the air-magnetic type or of the air-blast
type. Due to cost and size restrictions, air-blast
breakers are not normally used in medium-voltage
drawout switchgear construction. In recent years,
most medium-voltage drawout switchgear employed air-magnetic breakers. However, due to
cost, size, and noise limitations, vacuum and SF6
circuit breakers are replacing air circuit breakers in
medium-voltage drawout switchgear.
(2) The contacts of vacuum circuit breakers are
hermetically-sealed in a vacuum chamber or
“bottle”. The vacuum in a new bottle should be
about 10-8 Torr, and should be at least 10-4 Torr for
proper operation. One Torr equals 760 millimeters
of mercury. Vacuum interrupters are much smaller
and quieter than air circuit breakers, and require no
arc chutes. Vacuum circuit breakers in drawout
switchgear mounting are available in a variety of
continuous current and MVA ratings at 5kV to
15kV.
(3) Sulfur hexaflouride, SF6, is a nonflammable,
nontoxic, colorless, and odorless gas, which has
long been used in high-voltage circuit breakers.
Now, SF6-filled-interrupter circuit breakers are
3-7
TM 5-811-14
available in drawout switchgear for 5kV and 15kV
applications. Like vacuum interrupters, the circuit
breaker contacts are immersed in a hermeticallysealed bottle filled with SF6 gas. SF6 circuit breakers
in drawout switchgear mounting are available in a
variety of continuous current and MVA ratings.
c. EMI/RFI considerations. With today's increasing use of sensitive, solid-state devices, the effects of Electro-Magnetic Interference (EMI) and
Radio-Frequency Interference (RFI) must be considered. Solid-state devices, due to their many advantages, are rapidly replacing the rugged electromechanical devices previously used. One disadvantage of solid-state devices, however, is their sensitivity to power source anomalies and electrostatic
and electromagnetic fields. Recent developments in
the design and packaging of solid-state devices
have incorporated effective shielding techniques.
However, the designer must still evaluate the environment and ensure that additional shielding is not
required. Equipment and devices must comply with
MIL-STD-461.
d. In formation needed for coordination. The
following circuit breaker information is required for
a coordination study:
(1) Circuit breaker continuous current and
frame rating.
(2) Circuit breaker interrupting rating.
(3) Circuit breaker time-current characteristic
curves.
e. Circuit breaker ratings. Standard voltage and
current ratings for circuit breakers may be found in
appendix D. To meet UL requirements, molded
case circuit breakers are designed, built and
calibrated for use in a 40 degrees C (104 degrees F)
ambient temperature. Time-current characteristic
trip curves are drawn from actual test data. When
applied at ambient temperatures other than 40
degrees C, frequencies other than 60 Hz, or other
extreme conditions, the circuit performance
characteristics of the breaker may be affected. In
these cases, the current carrying capacity and/or
trip characteristics of the breaker may vary.
Therefore, the breaker must be rerated.
(1) Since thermal-magnetic circuit breakers are
temperature sensitive devices, their rated continuous current carrying capacity is based on a UL
specified 40 degrees C (104 degrees F) calibration
temperature. When applied at temperatures other
than 40 degrees C it is necessary to determine the
breaker's actual current carrying capacity under
those conditions. By properly applying manufacturer' s ambient rerating curves, a circuit breaker’s
3-8
current carrying capacity at various temperatures
can be predicted.
(2) Application of thermal-magnetic circuit
breakers at frequencies above 60 Hz requires that
special consideration be given to the effects of high
frequency on the circuit breaker characteristics.
Thermal and magnetic operation must be treated
separately.
(a) At frequencies below 60 Hz the thermal
rerating of thermal-magnetic circuit breakers is
negligible. However, at frequencies above 60 Hz,
thermal rerating may be required. One of the most
common higher frequency applications is at 400 Hz.
Manufacturer's rerating curves are available.
(b) At frequencies above 60 Hz, tests indicate
that it takes more current to magnetically trip a
circuit breaker than is required at 60 Hz. At frequencies above 60 Hz, the interrupting capacity of
thermal-magnetic breakers is less than the 60 Hz
interrupting capacity.
(3) When applying thermal-magnetic circuit
breakers at high altitudes, both current and voltage
adjustments are required. Current rerating is
required because of the reduced cooling effects of
the thinner air present in high altitude applications.
Voltage rerating is necessary because of the
reduced dielectric strength of the air. Refer to
ANSI C37-13 and ANSI C37-14 for specific
rerating factors to be applied at various altitudes.
(4) Trip curves provide complete time-current
characteristics of circuit breakers when applied on
an ac systems only. When applying thermal-magnetic circuit breakers on dc systems, the circuit
breaker's thermal characteristics normally remain
unchanged, but the manufacturer should be consulted to be certain. The magnetic portion of the
curve, on the other hand, requires a multiplier to
determine an equivalent dc trip range. This is necessary because time-current curves are drawn using
RMS values of ac current, while dc current is
measured in peak amperes. Additionally, the X/R
ratio of the system as seen by the circuit breaker
will affect its dc rating. When a circuit breaker
opens a dc circuit, the inductance in the system will
try to make the current continue to flow across the
open circuit breaker contacts. This action results in
the circuit breaker having to be derated.
Furthermore, some circuit breakers require the ac
waveform to pass through a current zero to open
the circuit. Since dc does not have current zeros,
the circuit breaker must be derated. For dc
applications the manufacturer should be contacted
for derating requirements.
f. System X/R ratio. Normally, the system X/R
ratio need not be considered when applying circuit
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breakers. Circuit breakers are tested to cover most
applications. There are several specific applications,
however, where high system X/R ratios may push
short-circuit currents to 80 percent of the shortcircuit current rating of standard circuit breakers.
These applications are listed below.
(1) Local generation greater than 500kVA at
circuit breaker voltage.
(2) Dry-type transformers, 1.0 MVA and
above.
(3) All transformer types, 2.5 MVA and above.
(4) Network systems.
(5) Transformers with impedances greater than
values listed in the ANSI C57 series.
(6) Current-limiting reactors in source circuits
at circuit breaker voltage.
(7) Current-limiting busway in source circuits
at circuit breaker voltage.
If the system X/R ratio is known, multiplying factors from various references can be used to determine the circuit breaker short-circuit current rating.
If the system X/R ratio is unknown, the maximum
X/R ratio of 20 may be assumed and the
appropriate multiplying factor used.
g. Circuit breaker application. Molded-case circuit breakers, power circuit breakers, and insulatedcase circuit breakers should be applied as follows:
(1) Molded-case circuit breakers have traditionally been used in panelboards or loadcenters
where they were fixed-mounted and accessible.
Low-voltage power circuit breakers, on the other
hand, were traditionally used in industrial plants and
installed in metal-enclosed assemblies. All power
circuit breakers are now of the drawout-type
construction, mounted in metalclad switch-gear.
Therefore, molded-case breakers should be used in
fixed mountings, and power breakers should be
used where drawout mountings are employed.
(2) Since power breakers were traditionally
used in metal-enclosed assemblies, they were rated
for 100 percent continuous duty within the assembly. On the other hand, molded case breakers were
traditionally used in open air. When used in a metal
enclosure, molded-case breakers had to be derated
to 80 percent of continuous rating. Molded-case
breakers are now available at 100 percent rating
when installed in an enclosure.
(3) Power breakers have traditionally been applied where selectivity was very important, thus
requiring high short-time ratings to allow downstream devices to clear the fault. Molded-case
breakers were, instead, designed for very fast operations. Fast opening contacts under high short-cir-
cuit current conditions resulted in molded-case
breakers having higher interrupting ratings than
power breakers.
(4) An insulated-case circuit breaker is somewhat of a hybrid circuit breaker which incorporates advantages of both the molded-case and
power circuit breaker. However, an insulated-case
breaker is not a power breaker, and should not be
applied as such. Insulated-case breakers are not
designed and tested to the same standards as power
breakers. An insulated-case breaker is essentially a
higher capability molded-case breaker. All
commercially available insulated-case breakers are
100 percent rated.
(5) Molded-case or insulated-case breakers
should be used in noncritical, small load applications with high interrupting requirements. Power
breakers should be used in critical applications
where continuity of service is a requirement. For
overlapping applications, designer judgment
should be based on factors discussed in this TM.
Refer to table 3-3 * for circuit breaker application
comparisons.
3-6. Protective relays
Protective relays are classified according to their
function, and there are a wide variety of protective
relays available. The overcurrent relay, for example,
monitors current and operates when the current
magnitude exceeds a preset value.
__________
*Adapted from Application Considerations from Circuit
Breakers-Choosing the Right Type for Specific Applications by
S.H. Telander, Consulting-Specifying Engineer Magazine, July,
1987.
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a. Overcurrent relay. The most common relay
for short-circuit protection is the overcurrent relay.
These relays are much more sophisticated than the
simple thermal overload relays discussed previously
for motor applications, and have a wide range of
adjustments available. Electromagnetic attraction
relays may be ac or dc devices and are used for
instantaneous tripping. Electromagnetic induction
relays are ac only devices. Electromagnetic
attraction and induction relays, like all electromechanical devices, are simple, rugged, reliable,
and have been used successfully for years. However, solid-state electronic relays are rapidly replacing the electromechanical types. Solid-state
relays require less panel space and exhibit better
dynamic performance and seismic-withstand capability. Additionally, solid-state overcurrent relays
are faster, have more precisely-defined operating
characteristics, and exhibit no significant overtravel.
As in the case of circuit breakers, electromechanical
relays will continue to find applications in harsh
environments. Overcurrent relays have a variety of
tap and time dial settings. Typical relay ratings are
shown in appendix D, and typical over-current relay
time-current characteristic curves are shown in
appendix C.
b. Relay device function numbers. Protective
relays have been assigned function numbers by
IEEE that are used extensively to specify protective
relays. A partial list of relay function numbers is
included in appendix E.
c. Instrument transformers. Protective relays
will always be associated with medium-voltage and
high-voltage circuits, involving large current magnitudes. Therefore, current transformers (CT) are
required to isolate the relay from line voltages and
to transform the line current to a level matching the
relay rating. CTs are normally rated 5A on the
secondary with a primary rating corresponding to
the requirements of the system. Potential or voltage
transformers (VT) are single-phase devices, usually
rated 120V on the secondary with primary rating
matched to the system voltage.
(1) CT burden is the load connected to the secondary terminals. Burden may be expressed as voltamperes and power factor at a specified current, or
it may be expressed as impedance. The burden
differentiates the CT load from the primary circuit
load.
3-10
(2) Residually-connected CTs and core-balanced CTs are illustrated in chapter 5. Residuallyconnected CTs are widely used in medium-voltage
systems, while core-balanced CT's form the basis of
several low-voltage ground-fault protective
schemes. Relays connected to core-balance CTs
can be made very sensitive. However, corebalanced CTs are subject to saturation from
unbalanced inrush currents or through faults not
involving ground. High magnitude short-circuit
currents may also saturate core-balance CTs thus
preventing relay operation.
d. EMI/RFI With today's increasing use of
sensitive, solid-state devices, the effects of ElectroMagnetic Interference (EMI) and Radio-Frequency
Interference (RFI) must be considered. Solid-state
devices, due to their many advantages, are rapidly
replacing the rugged electromechanical devices
previously used. One disadvantage of solid-state
devices, however, is their sensitivity to power
source anomalies and electrostatic and electromagnetic fields. Recent developments in the design and
packaging of solid-state devices have incorporated
effective shielding techniques. However, the
designer must still evaluate the environment and
ensure that additional shielding is not required.
Equipment and devices must comply with MILSTD-461.
e. New developments. Microprocessor-based relays are also becoming available which provide
multiple relay functions as well as metering, fault
event recording, and self-testing in a single enclosure. This system requires fewer connections and
less panel space than individual relays and associated peripherals.
3-7 Automatic reclosing devices
Automatic reclosing schemes should not be applied
where the load being protected is a transformer or
cable, since faults in these types of loads are usually
not transient in nature. Automatic reclosing
schemes applied to permanent faults in transformer
or cable loads may result in equipment damage and
personnel hazards. Additionally, automatic reclosing schemes should be guarded against in motor
circuits. If the system voltage is restored out of
phase, the motor windings, shaft, and drive
couplings may be damaged. Furthermore, reclosers
should be applied only on aerial distribution systems.